How To: Use CSS :after pseudo-elements to create simple overlays

Posted on 30 Apr, 2018

More and more in web design, we find ourselves putting text on top of images. More often than not, this is a dangerous game. Images have dynamic color and lighting and text for the most part is one color. This is often a nightmare for readability and accessibility.

This means we want to introduce an overlay to sit between the image and the text. Sometimes this darkens the background image enough for readability. Other times it’s a branding opportunity. Either way we need a simple CSS technique to introduce this sort of overlay.

Since I prefer not to introduce new markup for an embelishment, we’ll use the CSS :after pseudo-element.

The process looks something like this:

Create the simplest HTML for your area

Use a :before or :after element to create your banner

Fix z-index issues caused by absolute positioning

Experiment with mix-blend-mode for fun and profit

Step 1: All the markup you need, none of the bloat

In a banner, all we really want is the banner’s container and any content that banner needs to contain.

<sectionclass="banner"><h1>Hello World</h1></section>

In this example, we’ll just utilize a section container and an <h1>. If you added more content, it could be siblings to the <h1> or you could place all of your content in a content container of some sort to do any positioning.

A little CSS magic is happening here for the added height of the banner as well as the centering of the text. That’s not important for this demo, but if you’re curious, it exists in the CodePen.

Step 2: Add the overlay element dynamically with :after

Natively, CSS gives us the powerful :before and :after elements for adding stylistic content to the page that shouldn’t affect markup.

By apply :before or :after to an element, you can insert a dynamic element into the DOM before or after the selected elements children.

One important note, all pseudo-elements require a content CSS property to display. In our case, this will just be a blank string.

Now we have an element that is full-width and -height. To do this, we utilize absolute positioning, as we don’t want to affect the content flow of the document.

We make the overlay slightly transparent utilizing the opacity property.

In this example, I chose a fun gradient, but you could use a simple background color or even another image to overlay.

Step 3: Fix z-index issues

The keen-eyed observer would notice that something isn’t quite right in the example. Our friendly overlay is covering not just the background image, but also the text in the banner.

By using absolute positioning, we’ve actually put the overlay on top of the stacking context of our banner. To fix this, your overlay and your content will need to have a z-index applied to them. I usually give the overlay a 1 and my content 100.

.banner:after{...
z-index:1;}.banner>*{z-index:100;}

And with that we have a finished overlay.

Bonus step: Advanced overlays with blend modes

I’ve been toying with background blend modes for a little while now, but it blew me away when I discovered mix-blend-mode. This allows a developer to blend multiple elements together!

Use mix-blend-mode on your overlay and you’ve got some fun new combinations to try out.

The support for various blend modes are pretty weak in the Microsoft browsers, but you can still use them today with clever progressive enhancement. If you want them to be built in Edge, you can let Microsoft know about your passion here.

Until that time, let’s use @supports queries to make sure our code still respects our friends using Edge and IE. The above code removes the transparency from our overlay and lets the blend mode do it for us. Instead of removing it, let’s negate it behind a support query.

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